Intravascular radially expandable stent

ABSTRACT

An improved radially expandable stent formed from a fine wire bent into a serpentine flat ribbon which is wound around a mandrel into a cylindrical sleeve for mounting on a balloon catheter for transluminal insertion in a vessel such as a blood vessel is provided. A very small diameter fine platinum wire is used to form the basic cylindrical sleeve and it is welded to a pigtail of the wire forming the sleeve to provide longitudinal stability.

This is a continuation of application Ser. No. 619,010, filed Nov. 28,1990 now U.S. Pat. No. 5,161,547.

FIELD OF THE INVENTION

This invention relates to intravascular implants for maintainingvascular patency in human blood vessels. More particularly, thisinvention relates to a radially expandable stent made from a fine wireformed into a serpentine ribbon wound into a cylindrical shape forintroduction into a body vessel for balloon expansion therein in aradial fashion to support the wall of the vessel when in the expandedconfiguration. This invention is particularly useful in transluminarimplantation of a stent for use in the coronary angioplasty to preventrestenosis.

BACKGROUND OF THE INVENTION

The basic concept of stents has been known for a number of years.Various types of stents have been proposed and patented, includingself-expanding spring types, compressed spring types, mechanicallyactuated expandable devices, heat actuated expandable devices, and thelike. More recently, expandable sleeves have been proposed such as shownin U.S. Pat. No. 4,733,665 to Palmaz, issued Mar. 29, 1988. In this andother patents Dr. Palmaz suggested a series of metal sleeves which couldbe expanded by a balloon catheter through the elastic limit of the metalso as to permanently deform them into contact and support of theinterior surface of the blood vessel in question. Subsequently, patentsto Hillstead, U.S. Pat. No. 4,856,516 issued Aug. 16, 1989 and U.S. Pat.No. 4,886,062 issued Dec. 12, 1989 to Wiktor, have shown stents formedof a zigzag wire wound around a mandrel in a somewhat cylindricalfashion which can then be mounted on a collapsed catheter balloon andexpanded after introduction into the vessel by expanding the ballooncatheter. These prior art devices have been satisfactory for certaininstallations, but have been limited in the amount of support that canbe provided to the interior of the blood vessel wall and in some cases,to the ratio of expansion possible, and in others cases in the size ofthe profile presented for the transluminal insertion.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a stentthat overcomes the limitations of the prior art.

It is another object of the present invention to provide a radiallyexpandable stent that can be formed from very fine wire to present avery low profile for introduction into a blood vessel.

It is a further object of the present invention to provide a fine wirestent that is economical to produce and yet able to maintain the desiredshape and size in the expanded state after installation.

It is yet a further object of the present invention to provide aradially expandable stent that is longitudinally dimensionally stable.

It is still a further object of the present invention to provide aradially expandable stent with sufficient surface area to fully supportthe interior walls of a body vessel when inserted therein.

These and other objects of the present invention are accomplished in oneembodiment formed from a fine wire bent into a flat serpentine ribbonand wound around a cylindrical mandrel to form a cylindrical sleeve forapplication to a collapsed balloon catheter for transluminal insertionin a blood vessel and later expansion by inflation of the ballooncatheter at the desired site.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other and further objects of the present invention withadditional features and advantages accruing therefrom will be apparentfrom the following description shown in the accompanying drawingswherein:

FIG. 1 is an enlarged scale plan view of the first step of the formationof a fine wire into the ribbon of the present invention;

FIG. 2 is a view similar to FIG. 1 of the serpentine wire ribbon formedfrom the wire configuration of FIG. 1;

FIG. 3 is a view of the wire ribbon of FIG. 2 wound about a mandrel toform a helix; with the wire pigtail of the ribbon of FIG. 2 welded tothe helix;

FIG. 4 is a view similar to FIG. 3 showing the stent mounted about acollapsed balloon catheter inserted in a blood vessel; and

FIG. 5 is a view similar to FIG. 4 on a reduced scale showing theexpanded stent in position in a blood vessel for holding the bloodvessel in the open configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a stent in accordance with the presentinvention is formed by first taking a fine wire 10 having a diameter ofapproximately 0.004", preferably made from platinum and forming it intoa generally sinusoidal form, as shown in FIG. 1 in which approximatelyten cycles or segments per inch are formed in the wire. These waves canbe formed in any convenient manner, for instance as by bending about arack gear by running a corresponding spur gear over a wire laid alongthe rack.

As may be seen in FIG. 2, the next step is to take the wire of FIG. 1and to further bend the sinusoids into a flat band containingalternately inverted teardrop shaped elements or loops 13. Each elementshares a common side with its neighbor and includes a base 17 and a pairof arcuate shaped legs 19--19 that come together in touching contact atan apex 15. The apex of a loop will lie along one side edge of the band,such as edge 21 while the base of the loop lies upon the opposite sideedge 23 of the band. In this configuration, approximately forty loops 13per inch of ribbon are present and the height or "amplitude" of theloops is approximately 1/16". This is accomplished by mechanicallybending the partially formed loops of FIG. 1 up against each other intothe shape shown in FIG. 2.

The fine wire 10 used to form the basic flat ribbon 12 is a softplatinum wire that has been fully annealed to remove as much springmemory as possible. The straight wire before bending, being in the fullyannealed condition, will retain whatever shape it is formed into.

After the flat narrow serpentine ribbon 12 is formed, as shown in FIG.2, the ribbon 12 is wrapped about a mandrel 14 having a diameter of0.060" in a spiral or helix fashion with the edges of each helix wrap 16of the ribbon 12 basically touching the adjacent ribbon helix edges toform a wire sleeve 18. The number of circumferential sections 16 on themandrel will determine the length of the sleeve 18, and a typical stentof this type may have a length of approximately one and one-half inches.

According to the present invention, as the serpentine ribbon 12 of FIG.2 is wound on the mandrel 14 of FIG. 3 the free end 20 of the wire ofFIG. 1 is inserted through the helix, as may be seen in FIG. 3. Inactual practice, the ribbon 12 is wound about the mandrel 14 over top ofthe free end 20 of the wire 10. After the helix is formed to the desiredlength, the free end 20 extending through the helix is trimmed, andwelded smoothly to the final turn or end circumferential section of thehelix 16 so as not to present any increased profile and so as not topuncture or pierce the balloon catheter or the blood vessel into whichit is being inserted. The end turn of the helix is welded at 22 andintermediate welds such as 24 are formed to stabilize the length of thehelix. The first turn of the helix at the other end may also be weldedto the free end at 26 so that the overall length of the stent can beconstrained and maintained in the desired configuration.

The serpentine ribbon sleeve 18 is next placed about a collapsed ballooncatheter as shown in FIG. 4. In this configuration, the sleeve 18generally has a diameter in the neighborhood of 1.5 mm for insertioninto the blood vessels adjacent the heart.

In use, the stent is mounted on a balloon catheter as shown in FIG. 4and is inserted into the appropriate blood vessel. The stent is guidedto the desired location where there is occluding plaque 28 or a weakvessel wall or other imperfection requiring placement of a stent. Oncethe stent is properly located and verified by fluoroscopic or othermeans, the balloon catheter is inflated to radially expand theserpentine wire sleeve 18. As the balloon expands, it expands the tightclosed apex of each loop of the serpentine ribbon 12 from "touchingcontact" shown in FIGS. 2-4 to a spaced apart condition as shown in FIG.5. For instance, in a particular embodiment where the diameter of thestent on the collapsed balloon catheter was 1.5 mm, the stent has beenexpanded to 4 mm to 5 mm within the blood vessel. The space 30 betweenadjacent loops then increases to something approximating 0.0875" withthe loop dimension being approximately 0.025". Thus, what initially inFIG. 2 was a "wavelength" of 0.025", now becomes a "wavelength" of0.1125". This is an increase of 4.5 times and is perhaps one of the morecommon expansion ratios found with stents of this type. With the presentstent expansion of up to 8 mm or six times has been found to be entirelysatisfactory.

At the same time, the "amplitude" or width 34 of the ribbon 12 decreasessome 20% to 25% due to the lengthening of the helix wrap due to theincreased circumference of the expanded sleeve. Thus, as the helix 16 islengthened by stretching the helix about the increased circumference ofthe expanded stent, the adjacent loops 13 are separated by spaces 30 atthe same time the amplitude 34 of the individual helixes decrease. Also,the overall length of the sleeve 18 tends to decrease even to the pointof causing the free end 20 to bend between the welds 22, 24 and 26. Thepigtail 20 prevents extension of the overall length of the sleeve 18,but allows it to contract as the diameter increases. The length tends todecrease because the middle of the balloon, and hence the middle of thestent, expands the most, pulling the ends toward the center.

It will be seen that this action maintains good interior surface supportof the blood vessel by maintaining the close spacing of the wire loopsand helixes forming the sleeve.

The expanded condition of the stent is shown in FIG. 5 with the ballooncatheter having been removed and the back portion of the sleeve 18 shownin dotted lines for clarity of presentation. Even in this expandedconfiguration, however, it will be seen that there are ample turns ofwire spaced closely enough to fully support the inner surface of theblood vessel so as to prevent collapse of the plaque occluded vessel.With this finer "mesh" serpentine configuration, smaller diameter wirecan be used without losing the necessary support for the interiorsurface of the blood vessel, and thus the stent presents a lower profileduring introduction which increases the utility of the stent for smallerblood vessel usage. This "finer mesh" also results in a more flexiblesleeve which, together with the smooth uniform surface of the tightlywound serpentine wire ribbon of FIGS. 2 and 3, improves the ease oftransluminal insertion and facilitates proper implantation and locationof the stent. Since the wire pigtail has no sharp ends and the free endis welded to the loop of the helix, there are no sharp edges or pointsto tear or catch on the catheter balloon or the interior surface of theblood vessel, and thus the stent of the present invention can be morereadily manipulated to the desired location.

In prior art devices where the necessary surface support had to beachieved by heavier wire or a denser sleeve, it became difficult to flexthe sleeve so as to transit the convoluted blood vessels. When a looserwire configuration was employed, the stability of the stent wasdecreased and the ultimate efficacy of the implanted stent compromised.

Since the stent of the present invention is welded to the longitudinalwire at several locations, the longitudinal stability of the stent isgreatly increased over the prior art devices without creating a stiffand inflexible stent that cannot be manipulated around curves andcorners of the vessel into which it is to be introduced.

In some prior art applications, sleeves of platinum were objectionablebecause of its inherent high elastic limit such that it required extremepressures to expand and to hold it in the expanded configuration withoutcontraction sometimes causing insufficient support of the wall surfaces.With the serpentine construction of the present wire form, the elasticlimit of in the annealed platinum wire can easily be overcome and thedevice can be fully expanded radially to support the blood vessel withvery little pressure required from the balloon catheter. Thus, applicantis able to provide a stent which is more radiopaque than, for instance,stainless steel, without encountering the usual modulus of elasticityproblems with platinum. This allows good visibility during implantationand speeds the procedure of positioning the stent in the proper locationwithin the vessel.

Thus with the construction and configuration shown, I have provided astent having good flexibility, dimensional stability, minimalimpurities, very smooth surface, low profile and immunity to fatigue andcorrosion.

While this invention has been explained with reference to the structuredisclosed herein, it is not confined to the details as set forth andthis application is intended to cover any modifications and changes asmay come within the scope of the following claims.

What is claimed is:
 1. A radially expandable stent for intravascular implantation that includesa plurality of helically aligned circumferential sections including two end sections and a plurality of intermediate sections that define a cylinder having a longitudinal axis, said cylinder formed of a continuous wire with said circumferential sections being spaced along said axis in abutting contact, each of said circumferential sections having expandable segments that impart radial expandability to said sections whereby said sections have an unexpanded insertion circumference and an expanded implantation circumference that is greater than said insertion circumference, said expandable segments being in their unexpanded mode, teardrop shaped elements that are alternately inverted about said circumferential sections, each element containing a base and a pair of legs that come together at a common apex when the stent is in an unexpanded condition. said expandable segments being in their expanded mode, U-shaped elements that are alternately inverted about said circumferential sections, and one of said end sections having a free end that is passed back along the circumferential sections and is joined in a single straight run to the other end section to prevent axial expansion of the stent during radial expansion.
 2. The stent of claim 1 wherein the free end of said end section is joined along its straight run to a plurality of circumferential sections to hold said sections in abutting contact as the sections are radially expanded.
 3. The stent of claim 2 wherein the free end of said end section is joined to the circumferential sections by welds.
 4. The stent of claim 1 wherein the wire forming the cylinder is annealed.
 5. The stent of claim 1 wherein wire forming the cylinder is annealed platinum. 